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[Timo Kluck <580225@xxxxxxxxxxxxxxxxxx>]

I have the same bug on a HP TouchSmart tm2, with the added bonus that
the backlight problems also happen at startup every now and then.

The easiest way I have found to trigger this situation is to set
brightness to 0 on a terminal (as root)

echo 0 > /sys/class/backlight/acpi_video0/brightness

Any other brightness set in this way is applied correctly. However,
after setting 0, the backlight never comes back on.

A workaround I use is to change screen orientation. For some reason, the
command

xrandr -o right

(followed by xrandr -o normal to get it back to normal) changes so many
things in the display settings that the backlight comes back on. I have
now assigned hotkeys to these commands, so that I can log in blindly,
then press the hotkeys, and have a working backlight.

I should also note that this problem does not occur after switching to
ATI graphics with the vga_switcheroo kernel module.

-- 
[gm45] Backlight sometimes stays off after resume on Lenovo X200
https://bugs.launchpad.net/bugs/580225
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